The present disclosure relates generally to high-voltage Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET), and more particularly to high-voltage MOSFETs integrated with a current-sense device.
A high-voltage MOSFET refers normally to a MOSFET capable of sustaining a drain-to-source voltage higher than 5 volt. It can be used for load switch, voltage conversion in power management, or power amplification.
Some high-voltage MOSFETs, also named power switches, are intended to conduct large current. FIG. 1A shows a circuit scheme for detecting current ID flowing through inductor LP, which usually happens in power conversion applications. High-voltage MOSFET 10 has a source electrode S connected to a current-sense resistor RCS1, a terminal voltage VCS of which can signal a control circuit the information of current ID since voltage VCS accurately reflects the magnitude of current ID. In FIG. 1A, all of current ID flows through current-sense resistor RCS1, which nevertheless could consume a large amount of power if current ID is huge.
FIG. 1B shows another circuit scheme for detecting current flowing through inductor LP. Integrated in high-voltage MOSFET are current-sense MOSFET NCS and main MOSFET NM, where current-sense MOSFET NCS has current-sense electrode CS connected to a ground line via current-sense resistor RCS2 but main MOSFET NM has source electrode S directly connected to a ground line. Both current-sense MOSFET NCS and main MOSFET NM are high-voltage devices. The theory of current mirroring could make the current flowing through current-sense MOSFET NCS in proportion to the current flowing through main MOSFET NM, such that voltage VCS output from current-sense resistor RCS2 substantially reflects current ID1 shared by both main MOSFET NM and current-sense MOSFET NCS. Only a very little portion of current ID flows through current-sense resistor RCS2, which according consumes little power.
FIGS. 1A and 1B also imply that both high-voltage MOSFETs 10 and 12 might become breakdown during a normal operation in order to release the power accumulated by inductor LP. Taking high-voltage MOSFET 12 as an example, when it is just switched from an ON state, conducting current, to an OFF state, not-conducting current, drain electrode D of high-voltage MOSFET 12 is going to be charged by current flowing through inductor LP, and the voltage at drain electrode D could become high enough to make high-voltage MOSFET 12 avalanche breakdown. Avalanche breakdown need not necessarily damage a device though. A rating for a power switch is called EAS, referring to “Energy during Avalanche for Single pulse” and meaning the maximum power of a single pulse that the power switch can sustain without damage. The higher EAS the stronger power switch.